Production of levan polymer from molasses by Zymomonas mobilis

Levan is a natural homopolysaccharide consisting of fructose units bound by β(2→6) fructofuranosydic bonds and it has a wide range of applications in various industries such as food, biomedical, chemical, medical and cosmetics. In this work, levan production performance of Zymomonas mobilis subsp. mobilis NRRL B-14023 cells cultivated in fermentation medium containing beet molasses pretreated with different methods was assessed at first. Beet molasses was pretreated with methods such as centrifugal clarification, sulphuric acid, potassium ferrocyanide, tricalcium phosphate and activated charcoal. Best pretreatment method was determined as centrifugal clarification, resulting in 17.13 g/L levan with 25% initial sucrose concentration and an initial pH of 5. During the analysis of levan produced in molasses medium, it was noticed that a clay-like material originating from molasses was causing interference. It is speculated that this material could be consisting of melanoidins, formed by Maillard reactions during sugar production. After that, levan production in synthetic medium by free and immobilized cells trapped in Ca-alginate in batch and continous fermentation systems was investigated. Fermentation temperature was 28 ºC and initial pH was 5 for both systems. A stirred-tank bioreactor with a production volume of 740 ml was manufactured and used for continous fermentation. In batch fermentation, free cells showed a better performance in levan production than immobilized cells (21.16 g/L and 14.05 g/L levan, respectively). In stirred-tank bioreactor, however, levan produced by immobilized cells (18.4 g/L) exceeded levan concentration reached by free cells (13.6 g/L) at 0.12 h-1 dilution rate. Maximum productivity values were observed at 0.21 h-1 dilution rate both for immobilized (3.465 g/L/h) and free cells (2.058 g/L/h). Thermal properties of levan produced in synthetic medium were investigated by Differential Scanning Calorimetry and compared with commercial levan. Melting point for commercial levan was found to be 202,25 ºC and meliting enthalpy was 199,90 J/g. Levan produced in synthetic medium had a melting point of 190,80 ºC and a melting enthalpy of 81,58 J/g.